1. Field of the Invention
The present invention is in the field of bicycles and tricycles, and more particularly relates to bicycle cranksets, and to construction methods and assembly means of such cranksets. For the purposes of this invention, a crankset is defined as a combination of a crankshaft, the crankshaft mounting bearings, and chain drive sprocket(s), or a means for mounting chain drive sprocket(s).
2. Prior Art
Bicycles have been used as a means of transportation and recreation for hundreds of years. During the entire existence of the bicycle, in order to increase performance, continual efforts have been underway to reduce the weight of the bicycle without reducing strength and/or stiffness. For optimum performance, a bicycle needs adequate stiffness so that pedaling energy is used for propelling the bicycle forward, rather than being absorbed in the flexing of structural components. It also needs to have adequate strength in order to prevent breakage under pedaling loads. Both cases preclude the possibility of reducing the weight of bicycles merely by reducing the amount of material used. However, in recent years, new lightweight, yet very strong, materials developed for the aerospace industry have been employed for use in bicycle structures. One example of these materials is a composite of carbon fibers bonded together with epoxy resin. This material, when properly designed and formed, can have the same strength and stiffness as an alloy steel structure, but only weigh xc2xd to ⅓ as much.
Carbon fiber/epoxy composites have been successfully used to make bicycle frames and rims that are lighter and stronger compared to metal frames and rims.
U.S. Pat. No. 4,704,919 to Durham discloses a two piece bicycle crankshaft. In the Durham device, rather than separating in the middle, one arm separates from one end of the spindle.
Attempts have been made at producing cranksets partly constructed from carbon fiber composite materials. Unfortunately, none of these cranksets have displayed any significant advantage over cranksets constructed of metal. One reason for this appears to be an incompatibility of carbon fiber/epoxy composites with the 3-piece type of crankshaft most modem lightweight, high performance bicycles employ, and upon which the carbon fiber crankshafts are based. In these type of crankshafts, a separate metallic spindle is mounted on bearings housed in the crank tube of a bicycle frame. Two crank arms, one on either side, are then attached to the spindle, normally by means of a mating four-sided taper and attachment bolt. The currently available carbon fiber crankshafts retain the metallic spindle and incorporates carbon fiber composites into the crankarms.
Carbon fiber composite crankarms are at a disadvantage when this type of connection between the arms and spindle is used, because it is not possible to economically produce a reliable four-sided mounting taper in carbon fiber. Therefore, this mounting taper on previous carbon fiber crankarms has been constructed from aluminum, and this aluminum portion is then adhesively bonded in some fashion to the carbon fiber.
The resulting structure is not significantly lighter nor stronger than standard forged aluminum crankarms, but it is much more expensive to manufacture. Because of this, existing carbon fiber crankarm designs have very small demand and sales, and are not a viable, profitable business investment.
Others have attempted to produce lightweight, yet strong and stiff bicycle cranksets by utilizing a two-piece metal design. In these designs, one crankarm is welded to the spindle, while the other arm is attached to the spindle with a splined connection. While this type of crank design is perhaps somewhat better than conventional three piece crankset designs, it still has limitations in that they are limited to a single material, such as steel, aluminum, or titanium.
U.S. Pat. No. 529,110 to Copeland discloses a two-piece bicycle crankshaft in which the two crankshaft halves are held together by an external threaded sleeve, rather than an internally housed tension bolt. The Copeland crankshaft is solid, with no mention made about utilizing a hollow crankshaft. Finally, the torque coupling illustrated is a tongue-and-groove type, rather than the dowel pins used in the present invention.
There accordingly remains a need for a design and construction which avails itself to optimum use of advanced composite materials.
In the present invention, a two-piece crankshaft has been developed in order to eliminate the heavy, inefficient tapered connection between the two crankarms and the spindle of conventional three-piece designs. The connection between the two pieces is located between the crankshaft supporting bearings, allowing the structure to be further optimized for the highest strength and stiffness for the least amount of weight.
In a preferred embodiment, the connecting point of the two pieces of the crankshaft is midway between the supporting bearings upon which the crankshaft is rotatably mounted to the bicycle frame crank tube. This arrangement allows for large diameter, tubular type structures to be used for not only the crank arms, but also for the spindle. Additionally, it allows for the spindle diameter to be approximately as large as the crank arm diameter, this in turn allows for each crankshaft half to be constructed from one continuous curved piece. The preferred embodiment includes a pair of one piece carbon fiber crankarms with integrated steel spindle portions bonded to them, and are referred to hereinafter as crankarm/spindle portions.
The inside face of each crankarm/spindle portion has a flattened end. Dowel pins are mounted on one of the flattened ends, and these pins engage with matching holes provided in the flattened end of the other crankarm/spindle portion. The dowel pins transfer the torque and bending loads from one crankarm/spindle portion to the other portion. The two crankarm/spindle portions are rigidly and detachably held together with a single bolt, which detachably yet securely fastens the flattened ends of the crankarm/spindle portions together.
The present invention also incorporates a novel pedal attachment fitting method at the other end of the crankarms, where the pedal attaches. The pedal attachment fitting is internally bonded in the tubular carbon fiber crankarm structure.
The preferred embodiment has several advantages over other crankset designs, including:
a) A lighter yet stronger and stiffer crankset;
b) A design which accommodates economical use of composite materials;
c) Rapid assembly and disassembly of the entire crankset;
d) Quick and easy removal of the one crankarm/spindle portion which carries the chain drive sprocket(s). This in turn allows for the easy change out of sprockets of various sizes;
e) Special tools are not required to separate the crankarm/spindle portions;
f) A single mold can be used for the construction of various crankarm lengths, rather than multiple molds. The carbon fiber crankarm can be cut off to the desired length, after which the pedal attachment fitting will then be bonded inside it.
The novel features which are believed to be characteristic of the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description of the accompanying drawings of the preferred embodiment of the invention. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.